The present invention relates to a method for preparing silalactones and to methods for preparing carboxyalkyl-substituted organopolysiloxanes therefrom.
Carboxyalkyl-substituted organopolysiloxanes are known from U.S. Pat. Nos. 2,723,987; 2,900,363; 3,119,855 and 3,391,177 and their utility as metal protectants and paper sizings is known from U.S. Pat. Nos. 3,755,071 and 4,011,362.
The methods disclosed in the art for the preparation of carboxyalkyl-substituted organopolysiloxanes typically comprise a hydrolysis reaction of either a cyanoalkyl-substituted hydrolyzable silane or a carbalkoxyalkyl-substituted hydrolyzable silane to prepare the carboxyalkyl-substituted siloxane unit, followed by a silanol condensation reaction and/or a siloxane equilibration reaction to provide the desired carboxyalkyl-substituted siloxane.
However, these methods for preparing carboxyalkyl-substituted organopolysiloxanes are not completely satisfactory in-as-much as the hydrolysis reaction of said cyano- or carbalkoxy-alkyl substituent is rarely complete and the final organopolysiloxane contains various amounts of residual radicals, such as cyanoalkyl radicals or carbalkoxyalkyl radicals. A method for preparing carboxyalkyl-substituted organopolysiloxanes which are free of such residual radicals is desired.
Silalactones are known from U.S. Pat. Nos. 2,589,456; 2,635,109; 2,963,500 and 3,395,167; however, the silalactones disclosed therein contain, or give rise to, triorganosiloxy units and are therefore not useful for preparing organopolysiloxanes which contain more than two carboxyalkyl substituents per molecule. Although the above-noted silalactone patents disclose that the triorganosiloxy-containing silalactones described therein are useful for preparing disiloxane dicarboxylic acids and various organo-functional organosiloxanes, no further teachings relative to carboxyalkyl-substituted organopolysiloxanes are given. Examples of said triorganosiloxy-containing or -forming silalactones of the art include ##STR2## wherein Me denotes the methyl radical.
U.S. Pat. No. 3,395,167 further discloses a process for preparing the triorganosiloxy-containing silalactone. Said process comprises heating an ester having the formula XR.sub.2 Si(CR.sub.2 ').sub.n COOA. The use of a catalyst to aid the reaction or the use of an ester containing two silicon-bonded X atoms was not contemplated in said patent.
Copending U.S. Ser. No. 565,075, filed on Dec. 23, 1983 and assigned to the assignee of this invention, discloses the preparation of silalactones by a process which comprises gently heating an ester in the presence of a halide salt catalyst selected from quaternary ammonium, phosphonium and pyridinium halide salts. It has now been discovered that certain tertiary amines or tertiary phosphines are effective catalysts for this process.